Leucocytozoon Simondi

Synonyms: Leucocytozoon anatis, Leucocytozoon anseris.

Disease: Leucocytozoonosis.

Hosts: Domestic ducks, domestic goose and many wild anseriform birds. Levine and Hanson (1953) tabulated reports of L. simondi from 23 species of wild waterfowl, including the greylag goose, white-fronted goose, Canada goose, wood duck, American pintail, green-winged teal, teal duck, blue-winged teal, falcated teal, mallard, black duck, baldpate, shoveller, scaup, lesser scaup, ring-necked duck, redhead, canvasback, American goldeneye, old squaw duck, hooded merganser, American merganser and redbreasted merganser.

Fallis, Pearson and Bennett (1954) transmitted L. simondi from domestic ducks to domestic geese, but failed to infect ruffed grouse, chickens, turkeys and pheasants with it.

Location: The gametocytes are in the lymphocytes, monocytes and also erythrocytes. Schizogony takes place in the liver, heart, brain, spleen, lungs, lymph nodes and pancreas.

Geographic Distribution: North America, Europe, Indochina.

Prevalence: This species is common in northern United States, Canada and other mountainous or hilly areas where cold, rapid streams permit suitable blackfly vectors to breed.

Morphology: The mature macrogametes and microgametocytes are more or less elongate, and 14 to 22 u long. Their host cells are ordinarily elongate, up to 45 to 55 u long, with their nucleus forming a very long, thin, dark band along one side and with pale cytoplasmic "horns" extending out beyond the parasite and the nucleus. In some cases, round macrogametes and microgametocytes in rounded host cells have been reported (Fallis, Davies and Vickers, 1951; Rawley, 1953; Cook, 1954); both types are mature and able to exflagellate. Briggs (1960) noted that there were approximately equal numbers of round and elongate forms in white Pekin ducks but that elongate forms were rare in Muscovy ducks, never constituting more than 5% of the total number. He suggested that this might be due to the influence of the host species.

The cytoplasm of the macrogametes is rather dark blue and the nucleus compact and red when stained with a Romanowsky stain. The cytoplasm of the microgametocytes is very pale blue and the nucleus diffuse and pale pink. The microgametocytes are more delicate and more subject to distortion than the macrogametes.

Species of Leucocytozoon in avian leucocytes

A good deal of controversy has existed as to the type of cell parasitized by L. simondi. The host cells of the mature gametocytes are so distorted that they cannot be recognized. Huff (1942) considered them to be lymphocytes or stages in transformation between them and monocytes. Levine and Hanson (1953) found young and developing forms only in lymphocytes or monocytes. On the other hand, Fallis, Davies and Vickers (1951) and Cook (1954) found very young forms in both lymphocytes and erythrocytes. Using the benzidine-peroxide stain for hemoglobin, Cook found no hemoglobin in the host cells containing mature gametocytes, but she found at least some hemoglobin in all of the 191 host cells she saw which contained developing gametocytes. She concluded that, while the ring stages may invade both erythrocytes and lymphocytes, they develop to maturity only in cells of the red blood series. Whatever the host cell may be, the gametes and gametocytes never contain hematin pigment granules.

Life Cycle: The life cycle has been studied by O'Roke (1934), Huff (1942), Fallis, Davies and Vickers (1951), Fallis, Anderson and Bennett (1956) and Cowan (1955) among others. Birds become infected when bitten by a blackfly vector. The sporozoites enter the blood stream, invade various tissue cells, round up and become schizonts.

Two types of schizont occur in the duck. Hepatic schizonts 11 to 18 u in diameter occur in the liver cells; they form a number of cytomeres which in turn form small merozoites by multiple fission.

Megaloschizonts 60 to 164 u in diameter when mature are found in the brain, lung, liver, heart, kidney, gizzard, intestine and lymphoid tissues 4 to 6 days after exposure. They are more common than the hepatic schizonts. The megaloschizonts develop in cells, possibly lymphoid cells or macrophages, within or outside the blood vessels. They contain numerous cytomeres and a large, conspicuous central body which may be either a primordium off of which the cytomeres  have budded (Cowan) or perhaps a hypertrophied host cell nucleus (Huff).

According to Cowan, spherical primary cytomeres are first formed. Their chromatin first diffuses and then proliferates to form peripheral clusters, which separate to form secondary cytomeres, which in turn multiply in the same manner. The multiplying cytomeres become smaller and more granular, their chromatin becomes more concentrated, and finally merozoite-like bodies are formed. These reproduce until the central body is greatly compressed and the megaloschizont membrane is ruptured, releasing the merozoites into the blood. Many thousands of bipolar merozoites are produced by each megaloschizont.

In addition to the hepatic schizonts and megaloschizonts, small structures thought to be schizonts were found by R. C. Ritchie (cited by Fallis, Anderson and Bennett, 1956) in the Kupffer cells of the liver of a duck killed 3 days after exposure.

On the basis of these observations, Fallis, Anderson and Bennett postulated the following life cycle: The first asexual generation occurs in the Kupffer cells of the liver. Some of the merozoites from these schizonts may develop into gametocytes; this explains the presence of a few large parasites in the blood 5 to 6 days after infection. Other merozoites from the first generation schizonts develop into hepatic schizonts, megaloschizonts and perhaps other Kupffer cell schizonts. Merozoites arising from megaloschizonts and hepatic schizonts develop into gametocytes which flood into the peripheral circulation beginning 6 to 7 days after infection. Some of these merozoites presumably develop into another asexual generation.

The development of the gametocytes in the blood cells has already been mentioned in the section on morphology. According to Chernin (1952), the gametocytes may disappear from the blood about 30 days after they first appear. Following this primary parasitemia, which begins in midsummer in northern Michigan, only an occasional parasite is seen in the blood during the fall and winter (O'Roke, 1934; Huff, 1942; Chernin, 1952a). With the development of sexual activity in the spring, gametocytes reappear in the blood and in some cases continue to be present thruout the summer.

It is clear from this account that schizogony continues in the internal organs for an indefinite, long time, altho at a much reduced rate. There are about 1000 times fewer gametocytes in the relapse phase than in the primary infection, and these adult birds are not seriously affected. However, they serve as the source of infection for the new crop of ducklings.

According to Chernin (1952a), the early season infections in ducklings are comparatively light, but the heavier pool of gametocytes provided by these primary infections in the first crop ducklings ensures the heavier and highly fatal infections which occur during midsummer.

The vectors of L. simondi are various species of blackflies (Simulium). O'Roke (1934) showed that S. venustum is the vector in Michigan. Fallis, Anderson and Bennett (1956) found that S. croxtoni and S. euryadminiculum are the important vectors during the early part of the blackfly season (May to June) in Ontario, while S. rugglesi is the important vector in late June and July.

In the blackfly's stomach (O'Roke, 1934; Fallis, Davies and Vickers, 1951; Rawley, 1953), 4 to 8 microgametes are formed within a few minutes by exflagellation from the microgametocytes. These fertilize the macrogametes to form a motile zygote or ookinete about 33 u long and 5 u wide. Ookinetes are present in the blackfly stomach 2 to 6 hours after ingestion of infected blood. They develop into oocysts both in the stomach wall and in the stomach itself.

The oocysts are 10 to 13 u in diameter. They can be found 2 to 3 days after infection, and complete their development 2.5 to 4 days after infection. They produce relatively few sporozoites compared with Plasmodium. The sporozoites are 5 to 10 u long, slender, with one end rounded and the other pointed. They break out of the oocysts and pass to the salivary glands, where they accumulate. Viable sporozoites can be found for at least 18 days after infective feeding.

Pathogenesis: L. simondi is markedly pathogenic for ducks and geese. The heaviest losses occur among young birds. O'Roke (1934) reported mortalities of 35%, 57% and 85% among young ducks in 3 different years in Michigan, but noted that the death rate among adults was very low. Knuth and Magdeburg (1922) and Stephan (1922) described serious outbreaks in young geese in Germany. According to Chernin (1952b), about 68% of the deaths in ducklings occur 11 to 19 days after exposure.

Briggs (1960) found that Muscovy ducklings were more resistant to L. simondi infections than white Pekin ducklings under conditions of natural exposure in Michigan. Altho both became readily infected, the mortality and number of sexual forms in the blood were much lower among the Muscovies than the white Pekins. In addition, deaths were delayed in the Muscovies.

The outstanding feature of an outbreak of leucocytozoonosis is the suddenness of its onset. A flock of ducklings may appear normal in the morning, may become ill in the afternoon, and may be dead by the next morning. Acutely affected ducklings are listless and do not eat. Their breathing is rapid and labored due to obstruction of the lung capillaries with schizonts. They may go thru a short period of nervous excitement just before death. Adult birds are more chronically affected. They are thin and listless, and the disease develops more slowly in them. If they die at all, it is seldom in less than 4 days after the appearance of signs. Ducklings which have recovered often fail to grow normally. Recovered birds, as mentioned above, remain carriers.

The principal lesions of leucocytozoonosis are splenomegaly and liver hypertrophy and degeneration. Anemia and leucocytosis are present, and the blood clots poorly. Cowan (1957) described the tissue reactions of infected ducks against the megaloschizonts. These include destruction by phagocytes and inflammatory cells, necrosis and possibly encapsulation.

Diagnosis: Leucocytozoonosis can be diagnosed by finding and identifying the gametocytes in stained blood smears or the schizonts in tissue sections.

Treatment: No effective treatment is known. Fallis (1948) found that quinacrine, sulfamerazine and chlorguanide were ineffective.

Prevention and Control: Prevention depends upon blackfly control - ordinarily a difficult task - or on raising ducks and geese under conditions which prevent them from being bitten by blackflies. In blackfly areas this means raising them in screened quarters. Blackflies pass readily thru ordinary, 16 mesh per inch window screening, and 32 to 36 mesh screen is needed to keep them out. Since this type of screening is expensive, a good grade of cheesecloth has been recommended for a single season's use.

This disease can be avoided entirely by raising ducks and geese in regions where blackflies do not occur in significant numbers. Since wild ducks and geese are reservoirs of infection for domestic birds, the latter should not be raised close to places where wild waterfowl congregate.